I removed a Kohler Magnum 18 from my GT and found a crack on one of the engine mounts that I would like to weld before I put it back in the tractor. Is the crankcase covers made of aluminium on those motors or something else?

According to the web magnesium will react to vinegar by foaming aluminum will not. Also magnesium filings can be ignited with a match aluminum not. Google "Identifying cast magnesium" don't take my word for it Don